python - 何时以及如何创建 sqlalchemy 链接表对象？

Question

当我在 sqlalchemy 中有多对多关系并且该链接表映射到一个类时，该 ORM 对象是如何以及何时创建的？例如，我无法通过 session.new 找到它。

这是一个简单的例子来解释它:

from sqlalchemy import create_engine
from sqlalchemy import Column, DateTime, String, Integer, ForeignKey, func
from sqlalchemy.orm import relationship
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker


Base = declarative_base()


class Department(Base):
    __tablename__ = 'department'
    id = Column(Integer, primary_key=True)
    name = Column(String)
    employees = relationship(
        'Employee',
        secondary='department_employee_link'
    )


class Employee(Base):
    __tablename__ = 'employee'
    id = Column(Integer, primary_key=True)
    name = Column(String)
    hired_on = Column(DateTime, default=func.now())
    departments = relationship(
        Department,
        secondary='department_employee_link'
    )


class DepartmentEmployeeLink(Base):
    __tablename__ = 'department_employee_link'
    department_id = Column(Integer, ForeignKey('department.id'), primary_key=True)
    employee_id = Column(Integer, ForeignKey('employee.id'), primary_key=True)


engine = create_engine('sqlite:///:memory:')
Base.metadata.create_all(engine)
Session = sessionmaker(bind=engine)


session = Session()
department = Department(name='department')
employee = Employee(name='employee')
department.employees.append(employee)
session.add(department)

# When does DepartmentEmployeeLink get created if ever as an ORM object?
print session.new

谢谢!

Answer 1

除非您显式查询该表，否则不会创建 DepartmentEmployeeLink 的实例。对于插入和查询 Employee 或 Department 实例，DepartmentEmployeeLink class 对象中的信息就足够了。无需为此创建实例。